Nitroso-modified Ziegler-Natta catalyst system

ABSTRACT

A modified Ziegler-Natta catalyst system, a method for preparing the catalyst system, and a process for polymerizing an olefin in the presence of the catalyst system are disclosed. The catalyst system comprises a titanium compound, an aluminum compound, and a nitroso compound. Improved polyolefin properties, such as high molecular weight, are obtained.

FIELD OF THE INVENTION

This invention relates to a modified Ziegler-Natta catalyst system. Thecatalyst system includes a nitroso compound, which influences polyolefinproperties such as molecular weight.

BACKGROUND OF THE INVENTION

Interest in catalysis continues to grow in the polyolefin industry. Manyolefin polymerization catalysts are known, including conventionalZiegler-Natta catalysts. To improve polymer properties, single-sitecatalysts, in particular metallocenes are beginning to replaceZiegler-Natta catalysts. Single-site catalysts typically require largeamounts of expensive activators such as methylalumoxane or salts ofnon-nucleophilic anions such as triphenylcarbeniumtetrakis(pentafluorophenyl)borate. It would be desirable to improvepolyolefin properties without the high cost of single-site catalysts andtheir activators.

Ziegler-Natta catalyst systems are well known in the art. UsefulZiegler-Natta catalysts include titanium compounds and theircombinations with aluminum compounds. It is known to support thetitanium compound with compounds such as silica or magnesium chlorideand considerable research has been done in this area. Known compositionsalso include an aluminum compound, sometimes referred to as acocatalyst. Trialkyl aluminums, dialkyl aluminum halides, and alkylaluminum dihalides are common cocatalysts.

It is known to add other compounds to a Ziegler-Natta catalyst system toinfluence catalytic properties. Various Lewis bases have been used; theyare often referred to as modifiers or electron donors. When the electrondonor is added during the preparation of the Ziegler-Natta catalystsystem it is sometimes called an “internal donor,” while those addedduring or immediately prior to the polymerization have been called“external donors.” A variety of electron donors have been disclosed (forexample, see U.S. Pat. No. 4,136,243). Common electron donors includeethers and esters (for example, see U.S. Pat. No. 5,968,865), but manyothers have been used. U.S. Pat. No. 5,106,926 gives examples ofsuitable electron donors as alkyl esters of aliphatic or aromaticcarboxylic acids, aliphatic ketones, aliphatic amines, aliphaticalcohols, alkyl or cycloalkyl ethers, and mixtures thereof withtetrahydrofuran being preferred. U.S. Pat. No. 4,927,797 discloses theuse of silane donors such as methylcyclohexyldimethoxysilane, and U.S.Pat. No. 6,228,792 discloses the use of 2,6-disubstituted pyridines aselectron donors. Sometimes two or more electron donors are used. U.S.Pat. No. 7,560,521 teaches a combination of a monofunctional donorselected from ethers, esters, amines, or ketones with a difunctionaldonor selected from diesters, diketones, diamines, or diethers. U.S.Pat. No. 6,436,864 discloses unsaturated nitrogenous compounds aselectron donors. An imine, a diimine, and a methoxymethylpyridine areused in the examples. A nitroso compound is not disclosed.

Chelating N-oxide ligands have been used in constructing single-sitecatalysts. For example, U.S. Pat. Nos. 6,498,221 and 6,875,829 usechelating N-oxides such as 2-hydroxypyridine N-oxide to synthesizecertain single-site catalysts. They are not used with Ziegler-Nattacatalyst systems. Nitroso compounds are not used. U.S. Pat. Nos.4,168,358 and 6,541,592 disclose the use of pyridine N-oxide as analternative to hydrogen or diethyl zinc to regulate molecular weight ina vanadium-based Ziegler-Natta polymerization.

U.S. Pat. No. 3,444,149 uses nitroso compounds to lower the molecularweight of polymers prepared with vanadium-based Ziegler-Natta catalystsystems. They disclose that the molecular weight may be remarkablyreduced and that liquid polymers may be obtained in this way.

The role of donors is not completely understood and remains a subject ofcontinued research. As polyolefin applications become more demanding,there is a continued need for improvements in catalyst systems. Despitethe considerable research that has been done in this area, apparently noone has studied nitroso compounds as a component in a titanium-basedZiegler-Natta catalyst system or contemplated that they might be used toincrease polyolefin molecular weight.

SUMMARY OF THE INVENTION

In one aspect, the invention is a modified Ziegler-Natta catalyst systemand a method for preparing it. In another aspect, the invention is aprocess for polymerizing an olefin in the presence of the catalystsystem. The catalyst system, which comprises a titanium compound, analuminum compound, and a nitroso compound, enables improved polyolefinproperties such as increased molecular weight.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a modified Ziegler-Natta catalyst systemcomprising: (a) a titanium compound; (b) an aluminum compound selectedfrom the group consisting of trialkyl aluminums, dialkyl aluminumhalides, alkyl aluminum dihalides, and combinations thereof; and (c) anitroso compound. The titanium compound can be any compound normallyeffective as a Ziegler-Natta catalyst. Preferred titanium compoundsinclude titanium halides such as titanium trichloride and titaniumtetrachloride, and titanium alkoxides such as titanium(IV) butoxide.

More preferably, titanium tetrachloride is used. When titaniumtetrachloride is used, it is preferably supported on or modified with amagnesium compound. Many magnesium compounds suitable for use insupporting or modifying the Ziegler-Natta catalysts are well known.Examples include magnesium chloride, alkyl magnesium halides, andmagnesium siloxanes. For additional examples, see U.S. Pat. Nos.4,298,718, 4,399,054, 4,495,338, 4,464,518, 4,481,301, 4,518,706,4,699,961, 5,258,345, 6,291,384, and 7,560,521, the teachings of whichare incorporated herein by reference.

Optionally, a Lewis base is included in the catalyst system. PreferredLewis bases are C₃-C₂₄ esters such as butyl acetate, diethyl phthalate,trimethyl trimellitate, and diethyl adipate and C₄-C₁₆ ethers such asdibutyl ether, glyme, and diglyme. More preferred Lewis bases are C₉-C₂₄esters such as diethyl phthalate, dioctyl isophthalate, and1,6-hexanediol bisbenzoate.

In one aspect, the titanium compound is a titanium halide supported onmagnesium chloride, and the Lewis base, if any, is present in a Lewisbase/Ti molar ratio less than 1. The supported titanium compoundpreferably has as a porosity (P_(F)) determined with the mercury methodhigher than 0.3 cm³/g, and typically in the range of 0.50-0.80 cm³/g.The total porosity (P_(T)) is usually in the range of 0.50-1.50 cm³/g,preferably from 0.60-1.20 cm³/g. The surface area measured by the BETmethod is preferably lower than 80, more preferably from 10 to 70 m²/g.The porosity measured by the BET method is generally from 0.10 to 0.50,preferably from 0.10 to 0.40 cm³/g.

Particles of the magnesium chloride-supported titanium compoundpreferably have substantially spherical morphology. Average diametersare preferably from 5 to 150 μm, more preferably from 20 to 100 μm.“Substantially spherical” particles are those wherein the ratio betweenthe major axis and minor axis is less than or equal to 1.5, preferablyless than 1.3.

The titanium compound preferably has the formula Ti(OR^(II))_(n)X_(y-n),wherein n has a value from 0 to 0.5, y is the valence of titanium,R^(II) is a C₁-C₈ alkyl, cycloalkyl or aryl radical, and X is halogen.Preferably, R^(II) is ethyl, isopropyl, n-butyl, isobutyl, 2-ethylhexyl,n-octyl, phenyl, or benzyl; X is preferably chlorine. TiCl₄ isespecially preferred.

One method suitable for preparing the spherical components mentionedabove comprises a first step in which a compound MgCl₂.mR^(III)OH,wherein 0.3≦m≦1.7 and R^(III) is a C₁-C₁₂ alkyl, cycloalkyl or arylradical, reacts with the titanium compound of formulaTi(OR^(II))_(n)X_(y-n).

The compounds are conveniently obtained by mixing alcohol and magnesiumchloride in the presence of an inert hydrocarbon immiscible with theadduct with stirring at the melting temperature of the adduct (100-130°C.). The emulsion is quickly quenched, and the adduct solidifies asspherical particles. Suitable methods for preparing the sphericaladducts are disclosed, e.g., in U.S. Pat. Nos. 4,469,648 and 4,399,054,the teachings of which are incorporated herein by reference. Anotheruseful method for making the spherical components is spray cooling,described, e.g., in U.S. Pat. Nos. 5,100,849 and 4,829,034.

For more examples of suitable titanium compounds and their methods ofpreparation, see U.S. Pat. Nos. 4,399,054 and 6,627,710, the teachingsof which are incorporated herein by reference.

The modified Ziegler-Natta catalyst system includes an aluminum compoundselected from the group consisting of trialkyl aluminums, dialkylaluminum halides, alkyl aluminum dihalides, and combinations thereof.Suitable aluminum compounds include triethylaluminum,tri-isobutylaluminum, diethylaluminum chloride, butylaluminumdichloride, and the like, and mixtures thereof. Trialkyl aluminumcompounds are preferred. Preferably, the molar ratio of the aluminumcompound to titanium compound is within the range of 0.5:1 to 500:1.

The modified Ziegler-Natta catalyst system includes a nitroso compound.By “a nitroso compound,” we mean nitroso compounds, dimers of nitrosocompounds, and mixtures of the monomeric and dimeric compounds. Thenitroso compound is an alkyl nitroso compound or a non-phenolic arylnitroso compound. Preferably, the nitroso compound is a C-nitrosocompound, i.e., a nitroso compound in which the nitrogen of the nitrosogroup is bonded to a carbon atom. Also preferred are nitroso compoundsin which the nitroso group is attached to a carbon that has no hydrogensattached. More preferably, the nitroso compound has the structure:

wherein R₁ is aliphatic C₁-C₁₆ hydrocarbyl, each R₂ is independentlyselected from H and C₁-C₁₆ hydrocarbyl; and wherein two adjacent R₂groups may be joined together to form a ring.

Some examples of suitable nitroso compounds are shown below:

Nitroso compounds can be prepared using a variety of methods known inthe art. The nitroso compounds are conveniently prepared by oxidation ofan amine or by a variety of nitrosation reactions (see Current Topics inMedicinal Chemistry (2005) 687 for a review of aliphatic C-nitrosocompounds and their synthesis). Other methods can be used. Dependingupon the substituents and the conditions, the nitroso compounds canexist predominantly as the monomer or as the dimer. For example,nitrosobenzene exists as the monomer at 20° C. in a 1 M benzene solutionwhile nitrosocyclohexane under the same conditions exists mainly as thedimer. For a discussion of the dimerization equilibria, see J. Org.Chem. 36 (1971) 3055 and references cited therein. Preferably, the molarratio of the nitroso compound to titanium compound is within the rangeof 1:1 to 50:1, more preferably from 10:1 to 30:1.

The modified Ziegler-Natta catalyst system is useful for polymerizingolefins. Preferably, the olefin is an α-olefin. Preferred α-olefins areethylene, propylene, 1-butene, 1-hexene, 1-octene, and mixtures thereof.More preferred are ethylene, propylene, and combinations of ethylenewith propylene, 1-butene, 1-hexene, or 1-octene. When ethylene ispolymerized in combination with another α-olefin, the modifiedZiegler-Natta catalyst system produces polyethylene with goodincorporation of the α-olefin. The amount of α-olefin incorporation willdepend upon the particular α-olefin and the amount added to thepolymerization. The level of α-olefin incorporation can be easilymeasured by FT-IR spectroscopy. Each molecule of α-olefin incorporatedgives one tertiary carbon atom.

The modified Ziegler-Natta catalyst system is useful for preparingpolyolefins with increased molecular weight. For some applications, apolyolefin with a high molecular weight, in particular, a high weightaverage molecular weight (M_(w)) is needed. M_(w) has a pronouncedeffect on melt flow properties. One measure of melt flow is melt index(MI) where the amount of polyolefin that flows through an orifice ismeasured as a function of time. Generally, MI decreases with increasingM. The modified Ziegler-Natta catalyst system is useful for preparingpolyolefins with a low MI. Polydispersity is the ratio of weight averagemolecular weight to number average molecular weight (M_(w)/M_(n)). Forcertain applications, a narrow molecular weight distribution (lowpolydispersity) is desired. It can be difficult to obtain lowpolydispersity with Ziegler-Natta catalysts, but the modifiedZiegler-Natta catalyst system is useful for preparing polyolefins withreduced polydispersity.

Optionally, hydrogen is used to regulate polyolefin molecular weight.The amount of hydrogen needed depends upon the desired polyolefinmolecular weight and melt flow properties. Generally, as the amount ofhydrogen is increased, the polyolefin molecular weight decreases and themelt index increases.

The polymerizations are normally conducted under pressure. The pressureis preferably in the range of 0.2 MPa to 35 MPa, more preferably from0.4 MPa to 25 MPa.

Many types of polymerization processes can be used, including gas phase,bulk, solution, or slurry processes. The polymerization can be performedover a wide temperature range. Generally, lower temperatures give highermolecular weight and longer catalyst lifetimes. However, because thepolymerization is exothermic, lower temperatures are more difficult andcostly to achieve. A balance must be struck between these two factors.Preferably, the temperature is within the range of 0° C. to 150° C. Amore preferred range is from 20° C. to 90° C.

Catalyst concentrations used for the olefin polymerizations depend onmany factors. Preferably, however, the concentration ranges from 0.01micromoles titanium compound per liter to 100 micromoles per liter.Polymerization times depend on the type of process, the catalystconcentration, and other factors. Generally, polymerizations arecomplete within several seconds to several hours.

The modified Ziegler-Natta catalyst system can be made by any suitablemethod; those skilled in the art will recognize a variety of acceptablesynthetic strategies. Each component can be separately added to thepolymerization reactor. Preferably, two or more components are combinedprior to addition. For example, the nitroso compound may be reacted withthe titanium compound prior to addition to the polymerization reactor.In one preferred method, the nitroso compound is reacted with thealuminum compound prior to addition to the reactor. More preferably, thenitroso compound is reacted with the aluminum compound and the reactionmixture is contacted with a titanium compound. This mixture is thenadded to the polymerization reactor. Most preferably, the nitrosocompound is reacted with the aluminum compound and the reaction mixtureis contacted with a titanium compound that has been modified by orsupported on a magnesium compound, especially magnesium chloride.

The following examples merely illustrate the invention. Those skilled inthe art will recognize many variations that are within the spirit of theinvention and scope of the claims.

Example 1 Modified Ziegler-Natta Catalyst System

A magnesium chloride and ethanol adduct is prepared following the methoddescribed in Example 2 of U.S. Pat. No. 4,399,054, but working at 2000RPM instead of 10,000 RPM. The adduct is treated thermally under anitrogen stream, over a temperature range of 50-150° C., until a weightcontent of 25% of ethanol is reached. In a 2-L four-neck flask, purgedwith nitrogen, TiCl₄ (1 L) is charged at 0° C. followed by the sphericalMgCl₂/ethanol adduct (70 g). The temperature is raised to 130° C. in 2hours and maintained for 1 hour. The stirring is discontinued, the solidproduct is allowed to settle, and the supernatant liquid is removed bysiphoning. Fresh TiCl₄ is charged to the flask, the temperature isbrought to 110° C. and maintained for 60 minutes. The stirring isdiscontinued, the solid product is allowed to settle, and thesupernatant liquid is removed by siphoning. The solid residue is washedonce with heptane at 80° C., five times with hexane at 25° C., driedunder vacuum at 30° C., and analyzed. The resulting solid contains 3.5%by weight titanium.

2-Methyl-2-nitrosopropane dimer (35 mg, 2×10⁻⁴ mole) is added to asolution of triethylaluminum (4×10⁻⁴ mole) in hexanes. The solution isstirred for 1 hour and 20 mg (2×10⁻⁵ mole Ti) of titanium tetrachloridesupported on magnesium chloride (prepared as described above) is added.The mixture is stirred for 30 minutes and used as described below in anolefin polymerization.

Example 2 Polymerization

Isobutane (1 L), 1-butene (20 mL), and 1M triethylaluminum solution inhexanes (4 mL) are added to a dry, stainless-steel, 2-L autoclavereactor. The reactor is heated to 80° C. and hydrogen is added from a300-mL vessel at 4.10 MPa to effect a pressure drop of 0.34 MPa. Thereactor is pressurized to 0.7 MPa with ethylene. The polymerizationreaction is started by injecting the modified catalyst system fromExample 1. The temperature is maintained at 80° C. and ethylene issupplied on demand to maintain the reactor pressure of 0.7 MPa. After 46minutes, the polymerization is terminated by venting the autoclave. Theresulting polyethylene sample is dried and tested.

Yield: 119 g. Activity: 7800 g polyethylene per g supported titaniumcompound per hour. By GPC, the polyethylene has a weight-averagemolecular weight (M_(w)) of 186,000 and a M_(w)/M_(n) of 5.8. Branching(by FT-IR spectroscopy): 4.3 tertiary carbons per 1000 carbons. Percentcrystallinity (by differential scanning calorimetry): 57%. Melt index(MI) by ASTM D-1238, Condition E: 0.33 dg/min. Rheological testing isperformed, and ER, an elasticity parameter measured according to ASTMD4440-95A (and as described in U.S. Pat. Nos. 5,534,472 and 6,713,585and in R. Shroff and H. Mavridis, J. Appl. Polym. Sci. 57 (1995) 1605),is 2.6.

Example 3

The polymerization of Example 2 is repeated, but with a catalyst systemmade by adding 2-methyl-2-nitrosopropane dimer (139 mg, 0.8×10⁻³ mole)to a solution of triethylaluminum (4×10⁻⁴ mole) in hexanes. The solutionis stirred for 1 hour and 20 mg (2×10⁻⁵ mole Ti) of titaniumtetrachloride supported on magnesium chloride is added. The mixture isstirred for 30 minutes and used in an olefin polymerization. The resultsare shown in Table 1.

Example 4

The polymerization of Example 2 is repeated, but with a catalyst systemthat uses nitrosobenzene (4×10⁻⁴ mole) as a replacement for2-methyl-2-nitrosopropane dimer. The results are shown in Table 1.

Comparative Example 5

The polymerization of Example 2 is repeated, but with a catalyst systemthat does not contain a nitroso compound. The system is prepared byadding 20 mg (2×10⁻⁵ mole Ti) of the same titanium compound to asolution of triethylaluminum (4×10⁻⁴ mole) in hexanes. The results areshown in Table 1.

Comparative Example 6

The polymerization of Example 2 is repeated, but with a catalyst systemthat uses N,N-diethylhydroxylamine (4×10⁻⁴ mole) as a replacement forthe nitroso compound. The results are shown in Table 1.

Comparative Example 7

The polymerization of Example 2 is repeated, but with a catalyst systemthat uses N,O-bis(trimethylsilyl)hydroxylamine (4×10⁻⁴ mole) as areplacement for the nitroso compound. The results are shown in Table 1.

Comparative Example 8

The polymerization of Example 2 is repeated, but with a catalyst systemthat uses 1-nitroso-2-naphthol (4×10⁻⁴ mole) as a replacement for2-methyl-2-nitrosopropane dimer. The results are shown in Table 1.

Comparative Example 9

The polymerization of Example 2 is repeated, but with a catalyst systemthat uses 2-nitroso-1-naphthol (4×10⁻⁴ mole) as a replacement for2-methyl-2-nitrosopropane dimer. The results are shown in Table 1.

TABLE 1 Polymerizations Time Branches/ Crystallinity Ex. (min) ActivityMl M_(w) M_(w)/M_(n) 1000 C (%) ER 2 46 7800 0.33 186,000 5.8 4.3 57 2.63 63 1600 0.11 248,000 6.6 4.6 55 2.6 4 95 2600 0.52 189,000 6.6 8.0 532.4 C5 30 8800 2.6 134,000 7.8 11.7 53 2.4 C6 63 4600 1.4 139,000 6.311.2 47 2.1 C7 50 6000 1.6 142,000 6.5 7.7 53 2.0 C8 44 6400 1.2 147,0007.1 7.8 56 2.1 C9 53 5200 0.9 138,000 6.8 8.0 51 2.1

Examples 2-4 show that the use of a nitroso compound provides increasedmolecular weight. The M_(w) of these polymers is higher than that of thepolyolefin made without nitroso compound (Comparative Example 5). Use ofa nitroso compound provides more than a 30% increase in M. There is alsoa significant decrease in MI to less than 0.9 for Examples 2-4 comparedwith a MI=2.6 without the nitroso compound (Comparative Example 5).Inspection of Comparative Examples 6 and 7 shows that this is anunexpected result; other similar compounds such asN,N-diethylhydroxylamine and N,O-bis(trimethylsilyl)hydroxylamineprovide little to no increase (less than 10%) in M_(w). ComparativeExamples 8 and 9 show that phenolic aryl nitroso compounds also havelittle effect.

The preceding examples are meant only as illustrations. The followingclaims define the invention.

1. A process for making polyethylene, comprising polymerizing ethylene and an olefin selected from the group consisting of 1-butene, 1-hexene, and 1-octene in the presence of a modified Ziegler-Natta catalyst system, wherein the catalyst system comprises: (a) a titanium compound; (b) an aluminum compound selected from the group consisting of trialkyl aluminums, dialkyl aluminum halides, alkyl aluminum dihalides, and combinations thereof; and (c) one or more aliphatic nitroso compounds; wherein the weight-average molecular weight (Mw) of the polyethylene is increased compared with that of a polyethylene produced using the catalyst system without the nitroso compound.
 2. The process of claim 1 wherein the titanium compound is titanium tetrachloride.
 3. The process of claim 1 wherein the nitroso compound has the structure:

wherein R₁ is aliphatic C₁-C₁₆ hydrocarbyl.
 4. The process of claim 1 wherein the molar ratio of nitroso compound to titanium is from 50:1 to 1:1.
 5. The process of claim 1 wherein the modified Ziegler-Natta catalyst system is prepared by a method comprising: (a) reacting a one or more aliphatic nitroso compounds with an aluminum compound selected from the group consisting of trialkyl aluminums, dialkyl aluminum halides, alkyl aluminum dihalides, and combinations thereof; and (b) contacting the reaction mixture from step (a) with a titanium compound.
 6. The process of claim 1 wherein the presence of the nitroso compound in the modified Ziegler-Natta catalyst system provides at least a 30% increase in the weight-average molecular weight of the polyethylene.
 7. The process of claim 1 wherein said one or more of said nitroso compounds comprises 2-methyl-2-nitrosopropane dimer. 